The time has come to order the wheels for my Firebird. After reading the sticky thread and researching, I picked my tires first: Fronts= 245/45/17, Rears= 275/40/17. Before you tell me to put 18's on it, I want M/T ET Street Radials & 275/40/17 is the largest size they come in.

It seems the general concensus is that 17x9.5" with a 5.5" Backspace is the correct wheel fitment for the rear tires. I've seen pics of this setup and it looks proportional and correct.

My fronts are what requires math. For the front tire width, I will need an 8" wide front wheel. I have an Ed Miller C4HD 13" front brake kit (small bearing hubs) which, according to Dean's post, adds .300" of positive offset to each side. Converting to millimeters, this is +7.620mm offset. Per the chart in the link on the wheel info sticky, for an 8" wide wheel, the closest backspacing would be 4.25", which is -6mm offset. Correct me if I'm wrong, but I would end up with a net offset of +1.62mm. This would mean the new wheels will stick out (toward the wheel well lip) less than 2mm more than stock (stock being a GTA or IROC front wheel which is 8" wide and zero offset, so 4" backspacing). Sound right? Sound like a good plan?

 

i don't think you will have any problems with that setup. i'm running the same size tire on 17x9's with 4.5" bs all around. the only thing is if you're lowered and hauling *** over a hilly road, you will hear the rubber touch every now and then ( time for coil overs!). the 5.5" bs on the rear will tuck the tires in a little though but will only look like your rears are out about 1/2" more than stock. mine stick right at the end of the quarters and fenders. your fronts will be the ones to worry about. aren't the 245/45's a little taller than the 275/40's? if you do experience any rubbing, just roll the fenders a little. sometimes a baseball bat and a friend watching will take care of that, but not always.

 

What are you basing your opinion on? 17x9 with 4.5" BS is totally different than my proposed front setup. Plus, are you running stock brakes or C4HD's? No offense, but I'm really looking for either (1) someone to check my math and tell me if I got it right, or (2) someone with my same front brake setup to tell me what they used.


No, in most tire brands they're both exactly the same... 25.7" However, M/T's spec on the ET Street Radial is 26.0", so the rears will actually be a little taller.

 

i'm just not going to worry about helping anyone anymore who obviously has the smarts to do an equation, but not solve it when they have it written in front of them.

 

WHOA... 4.25"bs is WRONG
The brakes you got will move the tire out .3" therefore you need to add that to the bs you want to bring the tire back in. If you want a 4.5" bs like everyone else on this board uses you will want to go with a 4.75"bs on the 8" rim. The 9.5" w/ 5.5bs is good
For reference take a look at Crazy Hawaiians wheels and read this post
https://www.thirdgen.org/forums/susp...t=10+inch+rims

 

remember, it's his math he is worried about.

 

OK, that makes sense. All I need to figure out is the point from which I need to start adding/subtracting (4.5" BS is the reference you suggest). Here's what I don't understand: A wheel with zero offset means that the mounting hub of the wheel is in the exact center of the wheel. However, on Skulte's site, it lists 82-92 Camaro 16x8" (front) Iroc wheels as "zero offset, 4.35" backspacing". That seems wrong to me. On an 8" wheel, shouldn't zero offset equal 4" backspacing (since the 4" mark would be the center of the 8" wheel)?

I was thinking that 4"BS was my target and I add/subtract from there to compensate for my brakes. In reading everyone else's posts, they're all using 4.5" BS on 8" wheels (with stock brakes). There's something I obviously don't understand I guess.

To mjustdie: Sorry if I pissed in your Cheerios, but you weren't giving me good info. Don't take it so hard, it's only the internet. If you want to take jabs at me, please PM me instead of posting in my thread.

 

kev
sounds to me if you're wheel track width overall is out .3" due to the brakes then adding .25 to the backspacing aka 4.5 stock to 4.75 new would work out nicely. Just my .02 don't kill me if i'm wrong, but i'm 99% sure that's how you should go. For the rears that sounds amazing and the M/T drag radials hook like a dream

 

[quote=Firebird 383;3251056] Here's what I don't understand: A wheel with zero offset means that the mounting hub of the wheel is in the exact center of the wheel. However, on Skulte's site, it lists 82-92 Camaro 16x8" (front) Iroc wheels as "zero offset, 4.35" backspacing". That seems wrong to me. On an 8" wheel, shouldn't zero offset equal 4" backspacing (since the 4" mark would be the center of the 8" wheel)?

Yes you are right, 0 offset should mean 4" bs on an 8 inch wheel. I honestly can't answer why Skulte says that is 4.35" bs and I don't wanna BS you. I always figure stuff out using backspacing ( never offset ) and I know that 4.5" bs is what everyone uses on the fronts with stock brakes.
However,
If you look at Crazy Hawaiians car in the link I posted earlier he has 8.5 inch fronts on stock brakes with 4.5bs which would stick out as much as an 8inch rim with 4" backspacing and it looks good too. And concequently would be very close to what yours would look like with an 8in rim with 4.5bs and your brake upgrade.

 

Your problem is that you are ASSuming zero offset on an 8" wheel is equal to 4" of backspacing; that is incorrect. Try a little "googling" and you will find the information you seek.

 

Google is how I got the wrong info the first time. I just re-googled and got a couple different answers. Apparently, the inner and outer beads are not figured into my equation above and that is what makes the backspacing on an IROC wheel 4.35" instead of 4"?

 

sheesh Kev, look at how cocky the n00bs on this site talk to the guys who have been here for years and years. I'd order up some 4.75bs rims and see what happens, don't mount a tire on them and if in doubt you could return them just in case. I think that should work out nicely on the front.

 

An 8" wide wheel will be 8" between the inside of the beads, or 4" backspace to the inside of the wheel lip. I usually add a bit for the bead thickness and tire bulge, since that's the true inside edge of the wheel/tire (as if you laid a straight-edge on it, and measured to the mounting surface of the wheel's hub). You're usually interested in the max inside of the wheel contacting some bodywork or suspension, which is why I measure it that way.

 

I can't do that-- I'm having them custom built, so no returns if they don't fit. Plus, I might buy a wheel/tire package so they'll arrive mounted and balanced.


That's what I found out when I re-Googled. There's so much wrong info on the web that you can't always trust the first thing you read, which is what I did on the offset equation. Thanks for helping set me straight.